[Feature Flag - Cleanup] OAuth access secret hashing

Summary

This issue is to cleanup the hash_oauth_secrets feature flag, after the feature flag has been enabled by default for an appropriate amount of time in production via #374588 (closed)

Owners

• Team: ~"group::authentication and authorization"
• Most appropriate slack channel to reach out to: #g_manage_auth
• Best individual to reach out to: @tachyons-gitlab
• PM: @hsutor

Stakeholders

Expectations

What might happen if this goes wrong?

Cleaning up the feature flag

• Create a merge request to remove <feature-flag-name> feature flag. Ask for review and merge it.
  • Remove all references to the feature flag from the codebase.
  • Remove the YAML definitions for the feature from the repository.
  • Create a changelog entry.
• Ensure that the cleanup MR has been deployed to both production and canary. If the merge request was deployed before the code cutoff, the feature can be officially announced in a release blog post.
  • /chatops run auto_deploy status <merge-commit-of-cleanup-mr>
• Close the feature issue to indicate the feature will be released in the current milestone.
• If not already done, clean up the feature flag from all environments by running these chatops command in #production channel:
  • /chatops run feature delete <feature-flag-name> --dev
  • /chatops run feature delete <feature-flag-name> --staging
  • /chatops run feature delete <feature-flag-name>
• Close this rollout issue.